removing text from richtextbox

removing text from richtextbox

I have richtextbox1 and want to remove the "USER1". from below sentence,
if sentence starts with ALTER TABLE and consists "USER1". what is the best
way to do that using C#É
ALTER TABLE "ENROLLMENT" ADD CONSTRAINT "ENR_STU_FK" FOREIGN KEY
("STUDENT_ID") REFERENCES "USER1"."STUDENT"("STUDENT_ID") ENABLE;
I want this:
ALTER TABLE "ENROLLMENT" ADD CONSTRAINT "ENR_STU_FK" FOREIGN KEY
("STUDENT_ID") REFERENCES "STUDENT"("STUDENT_ID") ENABLE;